The axial thrust of a gas turbine is the resulting force from aerodynamic forces and pressure forces which exert an axial force upon the rotor in the compressor and turbine, and also all pressure forces which act upon the rotor in the axial direction. The resulting thrust is absorbed by a thrust bearing. Gas turbines are typically designed so that they have a minimum thrust at no-load. The axial thrust increases in proportion to the load. In order to balance the axial thrust, an opposing force to the thrust balance can be applied to the axial thrust which increases with the load. Consequently, the maximum thrust which is to be absorbed by the thrust bearing can be reduced. The overall dimension and the power loss of the thrust bearing can be correspondingly reduced.
The thrust of turbines and compressors, and also the pressure forces which act upon the rotor in the axial direction, are determined by operating parameters, especially by the position of compressor stator blades and compressor discharge pressure, and also by the design. In this case, it is determined by the selected geometries, especially by the geometries of the blade passages and by the reaction degrees of the turbine stages. The operating parameters are dependent upon the desired process and operating concept of the gas turbine. The load-dependence of the thrust can no longer be changed once a design is selected.
The problem of thrust balance in gas turbines has been known for a long time and a large number of solution approaches were proposed in literature. In particular, different ways are known of balancing the axial thrust via pressure balance cylinders, and therefore of reducing the load upon the thrust bearing. Different methods have also been developed for controlling the thrust control by an opposing force in a gas turbine.
In U.S. Pat. No. 5,760,289, for thrust balance it is proposed to provide a balance piston downstream of the turbine and to apply compressed air to this pressure balance piston. A complex algorithm is required in order to control the pressure in the balance piston, and consequently to control the balancing force, independently of the operating state. Furthermore, a periodic calibration of the algorithm is proposed in order to compensate aging or possible modifications to the gas turbine.
Another embodiment of a pressure balance piston is represented in U.S. Pat. No. 4,653,267. In this case, the pressure balance piston in the center part, that is to say in the section which is located between compressor and turbine, is constructed as a twin-shaft arrangement. The axial force of the piston during normal operation is reduced by a second chamber which is exposed to pressure application with leakage air. Air can be discharged from this second chamber via a valve and as a result the pressure level in this chamber can be reduced. By changing the pressure level in the second chamber, the resulting axial force of the pressure balance piston is controlled. The advantage of this arrangement is that the air which is discharged for controlling from the second chamber can be reused for turbine cooling.
For the two proposals, additional constructional parts are needed for producing the pressure balance piston. For example casing components, shaft cover, turbine disks or turbine rings are understood as structural components in this case. Furthermore, compressed air, without output, is lost from the pressure balance piston via seals, or can only be used at a considerably lower pressure level. For accommodating the pressure balance piston, moreover, expensive installation space is taken up and, especially in the case of embodiments according to U.S. Pat. No. 5,760,289, an extension of the axis becomes necessary.
Another approach for reduction of the axial forces is set out in EP0447886. In the gas turbine design which is represented there, in which the shaft section which lies between the turbine and the compressor is a drum which is enclosed by a drum cover, and in which the annular passage which is formed between drum and drum cover undertakes the guiding of the cooling air which is tapped from the compressor to the end face of the turbine rotor, a considerable portion of the axial forces is applied as a result of the pressure on the first turbine disk. In EP0447886, the axial force is reduced by the static pressure being reduced upstream of the end face of the turbine rotor. This is achieved by cooling air inside the annular passage on the rotor side being deflected through a swirl cascade and being accelerated to the highest possible tangential velocity in the rotational direction of the rotor. In addition to the advantages of this embodiment, which are represented in EP0447886 itself, in comparison to the use of pressure balance pistons it is to be noted that no additional structural components or additional axial constructional length are required for the construction of a pressure balance piston. Furthermore, no compressed air is lost via pressure balance pistons. In the case of this embodiment, however, there is no way of controlling the axial thrust. This has the result that a considerable residual thrust is to be absorbed via the thrust bearing at full load or, at low load, that a thrust reversal is to be taken into consideration. Depending upon design and arrangement of the thrust bearing, increased vibrations can occur during a thrust reversal and in the most unfavorable case, at even lower load, an overloading of the counter-bearing can occur. Furthermore, with this design in the case of modifications to the gas turbine which have an influence upon the thrust, such as an upgrade as a result of a new compressor or a new turbine, no ways are provided of balancing this altered thrust.